Thanks Guys.....all fixed now
I reset the axle ratio in the SCT to "STOCK" - which I think turns it off.....
I then measured the actual rolling diameter of my rear tyre. 1 complete rotation = 78.5 inches.
There are 63,360 inches in 1 mile
So 63,360 divide by 78.5 = 807
Therefore - it ends up being 807 revs per mile.
When I used the online calculator to get the REVS per Mile :
http://www.gordon-glasgow.org/tirecalc.html
It gave me 783 Revs per mile for a 245 / 35 / 19 tyre
So I think that's where the error was.
Driving to work this morning after the change - the Speedo now shows 80 and the GPS shows 79......much better. Previously - it was out by about 8klm/h at 90.
I wonder if the trans shift points are also altered by this ? If the car thought it was doing 80 - but it was actually only doing 72 - it would have been shifting early - right ?